What are some potential pitfalls when working with date calculations in PHP?

One potential pitfall when working with date calculations in PHP is not taking into account timezones, which can lead to incorrect calculations or display of dates. To avoid this issue, always set the timezone explicitly before performing any date calculations.

// Set the timezone to the desired value before performing any date calculations
date_default_timezone_set('America/New_York');

// Perform date calculations here